In an impact driver, power driver, power drill, or other rotating tool that is provided with a rotating main shaft 43 having a machine tool retaining hole 42 into which a driver bit, drill bit, or other machine tool 41 such as the one shown in FIG. 1 is inserted, a clearance is conventionally provided between the machine tool retaining hole 42 and the machine tool 41 in order to insert various machine tools 41. In the drawing, the reference numeral 44 indicates a steel ball for securing the machine tool, 45 indicates a locking groove for locking the steel ball 44, 46 indicates a sliding tube for pushing the steel ball 44 into the machine tool 41, and 47 indicates a spring.
However, rattling occurs in the machine tool 41 as a matter of course when the clearance is present. Rattling of the machine tool 41 not only adversely affects working precision or efficiency, but also leads to damage and the like of the machine tool 41, and is undesirable.
Therefore, in order to eliminate this rattling, the external peripheral surface of the proximal end part of a machine tool 41 that is polygonal in cross-section may be pushed in the direction orthogonal to the axial direction of the machine tool 41 against the internal peripheral surface of a machine tool retaining hole 42 that substantially matches the cross-sectional shape of the proximal end part of the machine tool 41 to eliminate a prescribed portion of the clearance. However, when this method is used, the axial position A of the machine tool 41 is offset from the axial position B of the machine tool retaining hole 42, as shown in FIG. 2, run-out occurs in the machine tool 41, and satisfactory industrial work is impossible.